The present invention relates chiefly to the generation of an atmosphere formed at least partly by nitrogen and hydrogen which can be used, as one example, as a protective reducing atmosphere in a heat treating installation for items such as metals and alloys, and more particularly, wherein the atmosphere is obtained partly by cracking ammonia and partly by recovering the oxygen-contaminated atmosphere which has already passed through the installation.
Methods are known for producing non-oxidizing atmospheres, to be used, for example, in heat-treating operations performed in a metallurgical furnace wherein the non-oxidizing atmosphere after passing through the furnace is recycled for re-use. A method of this type is described in the Specification of French Pat. No. 819,373.
One such method employs a regenerative ammonia burner containing a combustion chamber which has a nozzle to which ammonia and primarily air are supplied, a coil in which the ammonia is cracked, and a heat-insulated catalysis chamber contiguous to the combustion chamber which contains a catalyst formed of a number of wire-gauze sheets. The air-contaminated atmosphere emerging from the metallurgical furnace is fed into the catalysis chamber, and the nitrogen and hydrogen resulting from the cracking of the ammonia in the combustion chamber also is fed into the catalysis chamber, to provide the additional, fresh gas needed to compensate for losses in the furnace. However, such a catalyst will only operate between 450.degree. and 600.degree. C., that is to say within a relatively narrow temperature range and at a relatively high mean temperature. It is therefore important to hold the wire-gauze sheets within this narrow range if the oxygen contaminant is to be satisfactorily removed. However, the temperature in the catalysis chamber is determined chiefly by the amount of air which infiltrates into the furnace. Where the amount of infiltrating is small the amount of oxygen in the catalysis chamber is decreased, resulting in lowering its temperature, which has to be compensated for by burning more fresh gas in the combustion chamber. Conversely, where the amount of infiltrating air is large the amount of oxygen in the catalysis chamber is increased, thus increasing the temperature in the catalysis chamber, and this increased temperature may be high enough to melt the wire-gauze sheets.
This type of method is rather difficult to carry out. It requires constant surveillance and entails a large number of manual operations to ensure that the apparatus performing this method is properly regulated. In many instances, the difficulties encountered lead to the abandonment of the atmosphere-recycling process and the used atmosphere simply is discharged, rather than re-used. This results in a considerable loss of gas and energy.